Characterization - Lipid Nanoparticles: Production, Characterization and Stability - page 56

is both relevant and measureable and therefore serves as the characteristic param-
eter of choice for nanoparticle charge (Hunter 2002 ).
The spatial distribution of ions, traditionally referred to as the electrical double
layer (EDL) around a charged surface determines its electrical state. The EDL is a
physical model consisting of two layers: a fixed layer and a diffuse layer (Fig. 4.8 ).
The fixed layer is a firmly bound layer while the diffuse layer is distributed within
the solution in contact with the charged surface. The diffuse layer has an increased
concentration of counter-ions. The fixed, bound layer has two surfaces of inter-
est—the genuine particle surface and the surface representing the centre of bound,
hydrated counter-ions, often referred to as the “Stern layer”. The ions beyond this
layer form the diffuse layer, also called the “Gouy” or “Gouy-Chapman layer”
(Delgado et al. 2007 ). The difference in the electrical charge within the EDL results
in a potential difference from the surface to the Stern layer to the diffuse layer.
The shear plane is then defined as the plane at which water molecules change
from being bound to the surface to being free to move, and is dependent on the
energy of mixing (shear). In the simplest case, this is considered a fixed position,
as defined in the model presented in Fig. 4.7 . The potential at this plane (the “Stern
potential”) is either equal to, or slightly higher (in magnitude) than the measured
potential at the shear plane which, in turn, is referred to as the zeta potential ( ζ ).
Fig. 4.8 Schematic
representation of electrical
double layer